In optical communications and measurement systems, optical signals are often transmitted across in a gap formed at two glass fiber ends. It is well known that signal losses can occur due to offset, angular misalignment, and surface defects at the terminal ends of the fibers. A conventional way of making connectors and other like fittings for holding fibers is to retain them with an epoxy resin. For example, to make a fitting which would hold the terminal ends of a multiplicity of fibers a plate would be cut with grooves suitable to receive the fibers, the fibers would be laid in the grooves, a cover plate would be added, and epoxy resin would be infiltrated to surround and fixedly retain the fibers in their grooves.
Similar techniques have been used to manufacture a specialized optical fiber device in which fiber array spacing and fiber end quality are especially critical. This is an optical position sensor which is comprised of a multi-channel binary encoded modulator plate placed between the transmit and receive ends of a fiber optic array. The components of the sensor comprise the fiber optic array transmitter, a light source coupled to the transmitter; a receiver precisely matching the transmitter fiber array with an equivalent number of channels, with each channel coupled to a photodetector; and a movable modulator plate which is connected to the article for which movement sensing is desired. The modulator plate is perforated and intercepts the light propagating through the transmit/receive gap. To make such a device, it is important that the opposing transmit and receive ends be identically aligned for optimal light transmission; it is also important that the spatial relationship between the multiplicity of fiber ends correlate exactly with the hole spacings in the modulator plate.
In miniature sized sensors which will produce a resolved linear motion of 0.127 mm, a 0.025 mm separation between channels must be implemented. In doing this, it is necessary to precisely align and space the fibers to form mirror image multi-channel fiber arrays (e.g. nine channels) on the transmit and receive sides. In utilizing a typical approach described above, a metal connector was made and cut in two, transverse to the axes of the fibers. The opposing cut surfaces were polished to make the transmit and receive sides. However, it was observed that there was variation in spacing between the individual fibers, due to slight lateral movement of some of the fibers within their U-shaped grooves. The amount of epoxy on one side of the groove was bigger than on the other side even though a minimal practical fit was obtained. Therefore, the fiber spacing did not exactly correspond with that of the modulator plate holes. Furthermore, there were voids at certain locations in the epoxy resin where the cut ends were exposed. This, combined with what appeared to be the inherent properties of the epoxy resin, resulted in glass fiber ends which tended to have either scratches (presumably due to accumulation of polishing grit and passage of an agglomerate across the surface of the fiber) and slight edge chips (presumably due to either voids or strength limitations for the epoxy under the action of the polishing.) Despite careful procedures, it was found very difficult to make a connector which had excellent light transmittance and excellent dimensional precision.
Also, certain optical fiber devices are intended for use in environments which may have either high temperature or high moisture content. For both these circumstances, epoxy resins present limitations. They tend to absorb moisture which can lead to fiber degradation at the interface between the fiber and the epoxy. They also are temperature limited and the epoxy resin will start oxidizing at elevated temperatures. In these instances and others, better hermetic seals are desired than epoxy resins allow.
Of course, there is considerable prior art relating to the making of terminal fittings for optical fibers. While the device described above is somewhat distinct, connectors for making splices have received considerable development. See particularly the article "Preparation of Optical Fiber Ends for Low Loss Tape Splices" by Chinnock, Volume 54, No. 3 (March 1975), pp. 471-477 and the article "A Vacuum-Assisted Plastic Repair Splice for Joining Optical Fiber Ribbons" by Cherin et al, Volume 58, No. 8, (October 1979), pp. 1825-1838, both in The Bell System Technical Journal, American Telephone and Telegraph Company, New York.
Connectors which do not use epoxy have been investigated. For instance, the article "Epoxyless Fiber Optic Connector Concepts for Single-Fiber Cables" by Esposito in Electronic Packaging and Production (January 1979), pp. 216-220, describes connectors which employ deformable plastic plugs. Also discussed therein is the use of solders including 60/40 tin-lead solder and 50/50 indium-tin solder. But while there was initial success, there was a lack of reproducibility and it was concluded that solder use was not promising. It was surmised that the effect was simply mechanical, in that upon solidification the solder shrunk and physically secured the fiber. In U.S. Pat. No. 4,119,363 to Camlibel et al a single optical fiber sealed in a housing by means of solder is also described. There it is said specifically that the solder upon solidifying and cooling squeezes against the fiber to form a hermetic seal. In the Camlibel et al fitting, the fiber is precisely aligned by the mechanical configuration of the housing. Related U.S. Pat. No. 4,252,457 to Benson et al mentions again the problems involved in getting actual bonds between metal and glass and discloses a method which involves swaging of a soft metal connector around the glass fiber. Thus, it can be seen that while metal solder joints are attractive in substitution of epoxy joints, they have been used in a limited manner and without great success.
Another limitation of connectors is that they cannot be constructed with disregard to the flexing which takes place at the point at which the fiber exited the connector at the side opposite the polished face. While a fiber is covered by a typical polymer coating up to a point at which it enters the connector, it was prone to breakage at that point due to flexing stress. While supplementary mechanical devices which grasp the jacket may be utilized, a simplified procedure was sought, preferably using the resin which bonded the fiber into the connective body. However, the epoxy resins that were regularly used did not tend to bond well to the polymer jacket.
Accordingly, there has been a need for improvement in making fiber optic array connectors in general and in the use of metallic solders in particular.